Update Travis CI to latest OTP19.1

Fix private includes when compiling in test profile

Helps cover extra cases.

When an include file is set in a private path (i.e. src/), the rebar3
compiler would not add them to the {i, Path} params -- only include/ and
the project root were being added.

This meant that when some extra source directories were added to the
compile job, such as test/ when running under the test profile, the
private include paths could not be shared with the test module.

This patch fixes the issues (and adds tests) for such a specific case by
adding all the configured include paths to the {i, Path} erl_opts
arguments, yielding successful compile runs.

Add support for behaviors, and not just behaviours

allow using an alternate regex to locate test modules during eunit runs

{`eunit_test_regex`, Regex}` will use the supplied `Regex` instead of
the default to locate tests in test dirs. note this matches only the
filename, not the path. the regex is applied to all test dirs, recursively

fixes #1331

compile + cover compile prior to analyzing coverdata

recompile all files when a parse transform given as an opt needs updating

there's no way to detect which files actually rely on a parse transform
passed to the compiler via the options (as opposed to `-compile(..)`
so if any parse transforms are in modules that need recompiling just
recompile the world

fixes #1328

don't compile when resetting coverdata

fixes #1327

update bbmustache 1.3.0

log error if a module failed to load_file, the user should be aware

Prevent crashing when `mv` warns and report warnings to the user instead.

In some cases, mv will throw a warning, while still moving the files
correctly and returning a 0 return code:

"mv: can't preserve ownership of ... Permission denied".

Atomically load changed erlang modules. Non-atomically load nifs. Reference to https://github.com/erlang/rebar3/pull/1317

opps. Infinite loop fixed.

https://github.com/erlang/rebar3/pull/1317 
In reference to with support to load erlang code atomically but load nifs non-atomically.

Print stacktrace in a more conventional way

Insert a newline before printing the stacktrace so that the term is
easier to read and copy. This is a more conventional way to print
traces, and is, for instance, the way it's done by make and python.

reset accumulated coverdata on writing out to disk

`cover:analyse/0` didn't exist pre-otp18

provider chains like `eunit, ct, proper` from misreporting cover
stats from providers later in the sequence

Fix crash when doing hash check with missing index

Cover excl mods

Specifically, when fetching an application where the expected hash is
unknown, the hash is validated from the hex index; when the index is
available, the hash is fetched fine and later inserted in the lock file.

However, if the index is not available, the call would simply crash.
This patch fixes thing so that instead, the index is refreshed before
giving up and failing.

- brings back former error handling and debug messages
- keeps the filtering of excluded mods and debug messages
- breaks up code into multiple functions and removes nesting

Equivalent trim_all in bin split for <17.x

The trim_all option used in binary:split/3 is not supported in 17.x.
This patch makes an equivalent operation by eliminating empty split
fragments. From the docs:

    trim
    Removes trailing empty parts of the result (as does trim in
    re:split/3.

    trim_all
    Removes all empty parts of the result.

The new expression is therefore equivalent to the old one, but with the
added benefit of compatibility.

Fixes #1275

upgrade relx to 3.21.0